Orthopedic implants must be strong, corrosion resistant and compatible with human tissue to provide a long useful life. Otherwise, an implant replacement operation with its attendant risks and discomfort, may be required during the lifetime of the patient. A long, useful life is most important when the recipient is young and active. To obtain useful longevity, the implant must be physically and chemically compatible with the body.
Ideally, an implant will have stiffness and elasticity identical to the healthy bone which it replaces. Many of the metal alloys, such as Ti-6A1-V4, typically used in prosthetic implants were developed for other applications, such as the aircraft industry. This alloy has an elastic modulus of about 120 GPa, far in excess of the roughly 17 GPa elasticity of healthy bone. Most other implant alloys have still higher moduli, for instance, 316 stainless steel has a modulus of about 200, and heat treated Co-Cr-Mo alloy has a modulus of about 240 GPa.
Excessive stiffness prevents the even distribution of forces along the contact surfaces between an implant and the surrounding bone. Tensile forces tend to concentrate in particular regions of the bone. In the unstressed regions of the bone, it has been found that the bone will tend to be absorbed into the body over time. This creates serious problems for the patient; notably, a weakening of the bone, a loosening of the bond between the implant and the bone, and possible shifting (i.e. sliding or rotating) of the implant relative to the bone.
Polymer composites have been tested as a possible alternative to metal alloy implants for load bearing applications such as hip joints and knee joints. For example, carbon reinforced poly(ether-ether-ketone) ("C/PEEK") and carbon reinforced triazine have been shown to have excellent stiffness and modulus for these applications.
Composite implants are presently manufactured by a process which begins with impregnating carbon fibers with the polymer. A block of composite material is built up from layers of preimpregnated carbon fiber, known as "prepregs." The block is then cut to the approximate shape of the implant. The final shape is obtained by abrading the surface with successively finer grades of sandpaper or grit. By selecting the appropriate base polymer and reinforcement, and the proper quantities of each, one can produce an implant that is "fine tuned" to its particular application. C/PEEK and carbon filled polysulfone are two such polymer composites.
Unfortunately, carbon reinforced composites like C/PEEK suffer from certain disadvantages which must be overcome before their physical properties can surpass those of metal alloys for use in load bearing implants. For instance, cutting and grinding of the composite material exposes carbon fibers at the surface of the implant. These carbon fibers are loose at the surface of the implant and are prone to fray off. These free carbon fibers may then migrate in the body of the patient or become trapped between the sliding surfaces of prosthetic joints. The abrasive action of these trapped fibers is known to cause rapid wear of the sliding surfaces.
Mechanical stresses and friction can also cause the composite lamina to separate. Delamination, as it is called, causes polymer particles and fibers to flake off and into the body where they too can become trapped between sliding surfaces of an artificial joint, or drift into body tissue.
When exposed to the in vivo environment, polymer composites are more electrochemically active than their metal alloy counterparts. Speculation has focused on the carbon reinforcement, which, it is believed, acts as an electron pathway allowing an electron exchange current to be set up between areas of the implant exposed to different ionic concentrations.
Thus, it can be seen that there are numerous difficulties that must be overcome before composites can become a favored alternative to metal alloys for use in load bearing implants.
Coating composite implants with a layer of pure polymer has been suggested as a way to overcome at least some of these difficulties. By isolating the carbon fibers underneath the surface of the implant, one is able to prevent these fibers from being trapped between sliding surfaces where they may act as an abrasive. Isolation under a nonconductive polymer coating will also prevent the carbon fibers from enhancing the electrochemical activity of the composite laminate.
Poly(ether-ether-ketone) ("PEEK") has been suggested as a coating for a C/PEEK implant. PEEK is well suited for use in an in vivo environment. It is relatively inert to corrosion or absorption in the body. Its high degree of crystallinity gives it high toughness as well as high compressive and tensile strength. Its crystallinity also makes PEEK resistant to solvation and gives it a high melting temperature. As a result of its solvation resistance, however, PEEK cannot be applied as a coating by conventional coating techniques. Only a strong acid can dissolve PEEK at room temperature. Consequently, the use of dipping or conventional spray coating would be unworkable because a solvent capable of dissolving the polymer would also tend to dissolve the implant. Furthermore, the use of solvent casting presents a problem when the workpiece is a prosthetic implant because it leaves behind volatile, potentially harmful substances which could leach into the body.
High temperature spray techniques appear to offer a promising and economical way to apply a coating of polymers, such as epoxy, polyester, polyethylene, polyamide and tetrafluoroethylene, to a metallic prosthetic implant. Kremith et. al., Plasma Spray Application of Plastic Materials, 12th National SAMPE Symposium, 1968, reports successful use of a plasma spraying technique to deposit such coatings on metal substrates. The investigators were able to obtain nonporous coatings at thicknesses of 0.012 to 0.015 inches, depending on the polymer. Surface adhesions were observed to be in the range of 311 to 1116 psi (much less than the 3000 psi surface adhesion achieved by use of the invention disclosed herein). The use of annealing to relieve internal stresses caused by the thermal expansion mismatch between the coating and substrate was not investigated.
Janowiecki et. al., Plasma-Sprayed High Temperature Polymeric Coatings, SAMPE Journal p. 40 (1968) reports the plasma spraying of polyimide and polyaryloxysilane coatings to metallic surfaces. Only a qualitative assessment of surface adhesion was made. The authors found the surface texture of their coatings generally rougher than that of low-melting polymer, applied by dip coating or conventional spray coating.
It is highly desirable to have a method of coating a polymer, polymer composite, ceramic or metal alloy implant with a highly crystalline polymer, such as PEEK, for the purpose of reducing the corrosion problems associated these implants and also to reduce the problem of wear debris formation in load bearing implant joints.